Kabul, Afghanistan (CNN) -- Suspected Taliban militants have executed a 7-year-old boy, accusing him of spying for the government, officials in southern Afghanistan said Thursday.

The execution took place Tuesday in the Sangin district of Helmand province, said Dawoud Ahmadi -- the provincial governor's spokesman.

In the past, militants have carried out similar killings of those accused of spying, Ahmadi said.

Three years ago, a 70-year-old woman and a child in the Musa Qala district of the province were executed following the same allegations, he said.

During a news conference Thursday, Afghan President Hamid Karzai said officials were looking into reports of the execution and said he condemned the act if it is confirmed to be true.

"I don't think there's a crime bigger than that that even the most inhuman forces on earth can commit," Karzai said. "A 7-year-old boy cannot be a spy. A 7-year-old boy cannot be anything but a 7-year-old boy, and therefore hanging or shooting to kill a 7-year-old boy ... is a crime against humanity."

"If this is true, it is an absolutely hiorrific crime," British Prime Minister David Cameron said during the news conference on an unannounced stop in Kabul. "If true, I think it says more about the Taliban than any book, than any article, than any speech could ever say."